CARBONIC ACID AND CARBONATES
Carbon. When any product of animal or vegetable life is strongly heated
in a vessel from which all air currents are excluded, a mixture of gases
and liquids is driven off, and a charred mass remains. This residue,
from whatever source obtained, is composed mainly of the element carbon.
It sometimes happens that a loaf of bread or a cake is left in the oven
and forgotten. In popular language it is then said to be “burnt to a
cinder”; in reality, the surface layers have been converted into carbon.
Carbonic Acid. If carbon is heated in an open vessel provided with a
good draught, it glows and in time disappears, because it combines with
oxygen to form an invisible gas, carbon dioxide or carbonic acid gas,
which, when dissolved in water, forms carbonic acid.
Compared with the acids which have been described in the foregoing
chapters, this is a very feeble acid; it changes the colour of litmus to
a wine red, not a bright pink; its taste is just pleasantly acid, and
its solvent action on metals and limestone is very small indeed. The
solution of the acid, obtained by passing carbon dioxide into water, is,
of course, very dilute, and it cannot be concentrated by evaporation,
since this only results in expelling the carbon dioxide from solution,
leaving pure water.
Soda Water. In the case of most gases, the weight which dissolves in a
given quantity of water is proportional to the pressure. This is true
for carbonic acid gas. Under a pressure of 4 atmospheres, the weight of
gas which dissolves is four times as great as under a pressure of one
atmosphere.
Soda water is water charged with carbon dioxide under pressure. This
pressure is maintained from the time it leaves the manufacturer to the
time it reaches the consumer by the strong walls of the syphon or
bottle. Immediately this pressure is released, the greater part of the
excess gas escapes, producing effervescence. It is, however, curious to
note that all the gas which ought to escape when the pressure is reduced
does not do so at once. If soda water is allowed to stand in an open
glass until it becomes “flat,” a brisk effervescence can be started
again by dropping a lump of sugar into the quiescent liquid. Soda water
remains supersaturated with gas for some time after the pressure has
been released.
Calcium Carbonate. The salts of carbonic acid are called carbonates.
Calcium carbonate is one of the most abundant substances in Nature. The
white cliffs of the east and south coasts of England, and those of
France across the intervening sea, are the exposed parts of enormous
beds of chalk or calcium carbonate. Whole mountain ranges in various
parts of the world are composed of limestone, which in some cases is
mainly calcium carbonate, and in others a mixture of this substance with
magnesium carbonate. Marble, whether white, black, or variegated, is
almost pure calcium carbonate, the differences of colour being due to
insignificant traces of iron and other foreign matter. In Iceland spar
and calc spar, sometimes called dog-tooth spar, we have two transparent
crystalline forms of this same substance.
Connected with the animal kingdom there are forms of calcium carbonate
no less varied in appearance. Egg shells are composed of this substance,
and so are oyster shells and the hard external coverings of some of the
lower animals. The mother-of-pearl lining of the oyster shell, and also
the pearl itself, are secretions of calcium carbonate. The beauty of the
last-named variety is due to the external form and to minute
inequalities of the surface, which cause the resolution of white light
into colours seen in the spectrum or in the rainbow. The coral reefs or
_atolls_ of the Southern oceans, which may be miles in breadth and
hundreds of miles in length, are all composed of calcium carbonate,
which a tiny marine animal has formed for its own support and
protection.
It is perhaps somewhat surprising at first to be told that all these
forms are composed of the same chemical substance, yet on this point the
evidence is definite and unmistakable. All the varieties dissolve
readily in dilute hydrochloric acid with effervescence caused by the
escape of carbon dioxide gas; moreover, if any of the purer forms, such
as pearl, marble, or Iceland spar, are heated to redness for some time,
they all lose about 44 per cent. by weight, leaving a residue which is
pure lime.
Quicklime. The making of lime from limestone or chalk is called lime
burning. The operation is carried out in a structure called a lime kiln,
which is usually a barrel-shaped vertical shaft surrounded by
substantial brickwork. There are two main methods of procedure, the one
continuous and the other intermittent. In the continuous process, the
kiln is filled up with limestone and fuel (generally coke) in alternate
layers. Combustion is started at the bottom and maintained by a
regulated draught. As the charge works down, the addition of limestone
and fuel is continued from the top, while the lime is removed from the
bottom of the kiln. The lime produced by this method has the ashes of
the fuel mixed with it. To avoid this, the more modern type of kiln has
four lateral fire grates outside the actual kiln.
For the intermittent method, a kiln is required which has a fireplace at
the bottom. Over this a rough arch is built of large pieces of
limestone, laid dry, and then the kiln is filled up with pieces of
limestone which decrease in size from below upwards. The fire is kindled
beneath the arch and urged by a regulated draught. The heating is
maintained for three days and nights, after which time the charge is
allowed to cool down.
Carbonic Acid Gas in Nature. Although the solvent action of carbonic
acid is very small compared with that of strong acids, it is
nevertheless great in comparison with that of water. This is shown
especially in its action on limestone, an action from which several
important consequences arise. Rain, as it falls through the air,
dissolves a little carbon dioxide and, although this is only an
exceedingly dilute solution of a very weak acid, its cumulative effect,
especially in limestone districts, is very great; it hollows out
enormous caves and causes the formation of those fantastic creations in
stone known as stalactites and stalagmites.
When a drop of water charged with carbonic acid gas falls on limestone,
it dissolves a little of that substance, forming calcium bicarbonate,
which may be regarded as a compound of calcium carbonate, carbon
dioxide, and water. Little by little, the solid rock is hollowed out and
a cave, or perhaps an underground watercourse, is formed.
Again, the drop of water charged with calcium bicarbonate may find its
way to the roof of a cave. As it hangs from the roof while it gathers
strength to fall, a little of the carbon dioxide escapes, and a minute
quantity of calcium carbonate is deposited. In this way, a stalactite
looking like an icicle in stone gradually grows downwards.
When the drop reaches the floor of the cave, a little time elapses
before it sinks into the ground; again a little carbon dioxide escapes,
and a small quantity of calcium carbonate is formed. Little is added to
little, and in the course of ages the stalagmite grows upward from the
floor and ultimately meets the stalactite to form a continuous column of
glistening crystallized calcium carbonate.
Hard and Soft Water. Water that is used for domestic or manufacturing
purposes is described as either hard or soft. Soft water produces a soap
lather almost at once; hard water forms at first a scum or curd which
has no detergent properties, and only after a time gives the soap lather
which is required. The difference is due to the relative amount of
dissolved solid contained in the water.
Only distilled water or rain water collected in the open country is
perfectly soft, for this is the only kind of water which on being
evaporated to dryness leaves no solid residue. In districts where the
underlying strata are composed of hard insoluble rock, such as granite
or millstone grit, the water contains very little dissolved matter and
is relatively soft. In a limestone or chalk country, water is very hard
and in many cases has to be softened either before delivery or before
use.
The chief impurities which cause hardness are the chlorides, sulphates,
and bicarbonates of magnesium and calcium. The chlorides and sulphates
are not affected in any way by boiling, and the hardness which is due to
them is said to be “permanent.” The bicarbonates, on the other hand, are
decomposed when the water is boiled, and then they cease to cause the
water to be hard. This part of the hardness is spoken of as “temporary”
hardness.
Let us now consider what calcium bicarbonate is and how it is formed. It
is a compound of calcium carbonate and carbonic acid, and is formed by
the solvent action of carbonic acid on limestone or chalk. The compound
is soluble in water; but when the solution is boiled, the carbonic acid
is broken up, carbonic acid gas is expelled from the solution, and
calcium carbonate is formed.
Temporary hardness is the more troublesome. In the first place, the
bicarbonates, especially that of calcium, often form the greater part of
the dissolved impurity. Moreover, when the water is boiled, although the
hardness is removed, the insoluble calcium carbonate is a source of
trouble, for it gradually settles down into the hard mass known as “fur”
in kettles and “scale” in boilers.
It is perhaps necessary at this point to emphasize the fact that matter
_suspended_ in water does not make it hard, and it is only matter which
is _dissolved_ which makes any difference in this respect.
Since the softening of temporary hard water by boiling has the
undesirable feature of introducing solid matter into the boiler, it is
customary now to treat this water chemically. The following is the
process most generally used. Quicklime or slaked lime is stirred into
the water until the mixture gives a faint brown coloration when a drop
of silver nitrate is added to a small test portion. Unsoftened water is
then added until a sample just ceases to give this test. The temporary
hardness has then been removed, and it is only necessary to allow the
suspended matter to settle.
The explanation of the method is as follows. The lime which is added
neutralizes the carbonic acid combined with the calcium bicarbonate, and
the result is the same as in the former case where this carbonic acid
was decomposed by heating, for calcium carbonate is thrown out of
solution.
For domestic purposes, water is softened by the addition of washing
soda. Since this reacts with all the calcium and magnesium compounds
forming the insoluble carbonates, all hardness, both temporary and
permanent, is removed.